The present invention relates to a novel method for the preparation of ultra-fine hollow glass spheres, which can be used, for example, as a filler of various kinds of lightweight composite materials from volcanic glass in the pyroclastic deposit, which is simply referred to as volcanic glass hereinafter.
It is a known technology as disclosed, for example, in Japanese Patent Publication 48-17645 and U.S. Pat. Nos. 3,752,685 and No. 3,904,377 that fine hollow glass spheres are prepared from a naturally occurring powdery volcanic glass by a heat treatment to cause expansion of the particles as a result of simultaneous gas evolution and softening of the particles. This prior art method has a limitation in the applicability in respect of the fineness in the particle size of the powdery starting material to be subJected to the heat treatment of the method. Namely, no ultra-fine hollow glass spheres can be obtained by this prior art method even by starting from a very finely pulverized material having a particle size of, for example, 20 .mu.m or smaller due to premature dissipation of the gas or water vapor before full softening of the particles.